Production or conversion of sound by or into electric currents



Jan. 17, 1928. 1,656,781

w. F. DUNN PRODUCTION OR CONVERSION OF SOUND BY OR INTO ELECTRIC CURRENTS Filed April 18. 1927 Patented Jan. 17, 1928. I

UNITED STATES PATENT OFFICE.

WALTER FREDERICK DUNN, OE CHARLTON, LONDON, ENGLAND, 'ASSIGNOR OF THREE- FOURTHS TO THOMAS ELIAS, OF WESTMINSTER, LONDON, ENGLAND.

PRODUCTION OR CONVERSION OF SOUND BY OR INTO ELECTRIC CUBRENTS.

Application filed April 18, 1927, Serial No. 184,740, and in Great Britain January 12, 1926.

This invention relates to the production or conversion of sound by or into electric currents, by electromagnetic apparatus adapted to operate in conjunction with the sound board or other vibrating element of a musical or other sound-producing instrument.

According to the invention, a relative movement between the armature and the magnet of the electro-magnetic apparatus involving variation in the magnetic field is caused. either mechanically by the vibrationv of the sound board or other vibrating element to transmit electrical impulses, or electrically by variations in the electric circuit to transmit-mechanical vibrations to the said vibrating element. Either the armature or the magnet, preferably the former, is positively attached directly or indirectly to the sound board or other vibrating element, but the armature does not touch. the pole pieces of the magnet, the gap between these arts. varying with the current and the vlbration's, so that chattering is avoided. The vibrations of the armature are preferably caused to take place in a linear direction at a right angle to the plane of the sound board. The armature being ads in one piece is free from internal vibrations, which in, a built-up member would be liable to cause distortion in the music transmitted.

In order that the said invention may be clearly understood and readily carried into effect, the same will be described more fully with reference to the accompanying drawings, wherein Figure 1 represents in horizontal section a device embodying this invention applied to the back of a piano,

Figure 2 illustrates a detail hereinafter mentioned.

Figure 3 represents diagrammatically the back of a piano fitted with apparatus exemplifying this invention.

A indicates a portion of the sounding board of a piano, B the bridge, C a bellybar or bracing strip. D the strings, E a portion of the outer casing or back frame of th piano, F a tube of metal. fibre. or other sui able material passing through the back frame E and secured'therein by any suitable means, as for instance by a ferrule f spaced from the frame E by rubber washers or by springs f and adjustable by nuts f working on the screw-threaded ends of bolts 7. In this tube F is a permanent magnet G; or alternatively it may be an electromagnet. This magnet has two pole pieces 9, g wound with coils of wire 9 insulated from the said pole pieces, the terminal connections of which coils are at g, with insulating strips g. The space between the bell-mouthed extension f of the tube F and the pole pieces and wire coils is preferably filled in with wax or a compound having similar properties in order to protect these parts from moisture and to prevent vibration of the casing. An insulating bush is indicated at f". To the belly-bar C an externally screwthreaded bush H is secured by ascrew h over which bush an internally screwthreaded socket h is attached. A ring h has a thin diametrical crossbar k (shown more clearlyimFigure 2) outside which an armature J is secured by a screw h" passing through the crossbarl into the socket k The ring if is retained in position opposite the flange f of the casing F by screws or bolts b the said parts It and 7 being spaced apart to prevent actual contact between the armature J and pole pieces 9, g, by means of a number of resilient washers surrounding the screws it. two of these washers being indicated at h, h; or alternatively a resilient ring perforated for the passage of the screws may be interposed between these parts it and f. The resilient spacing devices thus permit a slight rocking movement of the ring 72. to occur inresponse to the action of sound waves in the board B. The magnet and armature may of course be supported in the required positions in any other convenient way; for instance their relative positions with respect to the sounding board may be reversed. More than one magnet and armature may be employed for receiving electrical impulses, if found desirable.

no or more coils of wire 9' may be wound either on the magnet or the armature, or on both. The armature may be of any suitable shape, for example it may have projections at each side approachir the magnet, on which projections, or on other part or parts of the armature, one or more coils may be wound. Conductors may be brought from the terminals of an ordinary wireless receiving set to thesecoils, a better result being obtainable when an electric filter set is interposed with a loading resistance, this preventing resonance in the windings. The

filter set may be located inside the back of the piano near the bottom, as represented in Fi re 3, wherein K indicates the choke coi L the resistance, and M the condenser.

When the instrument is played in the ordinary manner, the vibrations of varying frequency thus set up in the sound board or the like cause varying relative movements between the armature and the pole piece or pieces, and consequently a variation in the electric current in the said circuit in correspondence with the fre uency of the said vibrations, and the sounds of the instrument can thus be reproduced at a distance on reconversion into sound vibrations of audible frequen by this or other a pliances. Converselp, y vibrations of t e soundboard cause by variations in the electric current, transmitted from any suitable appliance for converting sound vibrations into electrical im ulses, the instrument can be caused to emlt sound which may or may not be of the same nature as those ordinarily produced by the instrument, for example, reproducing sounds of speech or singing at a distance. For example in the latter case, a piano fitted with a device according to this invention may be connected with the output terminals of a wireless receiving set. In efiect the sounding board of the instrument is caused to perform the function of the diaphragm of a loud speaker, and as the said soundin board is of comparatively large area and possesses certain musical properties, a more correct rendering of the sounds transmitted is thereby obtained.

It will be observed that owing to the use of an armature as above described the sound ing board is ke t free, so that the piano can be played in t e ordinary wa when the wireless set is not being used; t 1e armature should not touch the pole pieces, as such contact might cause chattering but the gap between these parts varies with the current and the vibrations.

What I claim and desire to secure by Letters Patent of the United States is In a sound-producing instrument, a sound board, a socket adjustably connected with said sound board, an armature adjustably secured to said socket and arranged to vibrate conjointly with said sound board, a permanent magnet having pole ieces opposable to said armature, an electrical circuit comprising a coil wound on one of said pole pieces, a casing enclosing said magnet, and resilient spaclng means arranged between said armature and said casing.

WALTER FREDERICK DUNN. 

